Coordination versus Solvation in Al⁺(benzene)_<i>n</i> Complexes Studied with Infrared Spectroscopy

Abstract

Singly charged aluminum–benzene cation complexes are produced by laser vaporization in a pulsed supersonic expansion. The Al⁺(benzene)_<i>n</i> (<i>n</i> = 1–4) ions are mass selected and investigated with infrared laser photodissociation spectroscopy. Density functional theory (DFT) is employed to investigate the structures, energetics and vibrational spectra of these complexes. Spectra in the C–H stretching region exhibit sharp multiplet bands similar to the pattern known for the Fermi triad of the isolated benzene molecule. In the fingerprint region, strong bands are seen corresponding to the ν₁₉ C–C ring motion and the ν₁₁ out-of-plane hydrogen bend. The hydrogen bend is strongly blue-shifted compared to this vibration in benzene, whereas the ν₁₉ carbon ring distortion is only slightly shifted to the red. Computed structures and energetics, together with experimental fragmentation and vibrational patterns, indicate a primary coordination of three benzene molecules around the central Al⁺ cation. The <i>n</i> = 4 complex contains one second-sphere solvent molecule